Previously, as a spongelike structure, a variety of moldings are known. For example, there is a molding obtained by mixing a polymer with a blowing agent, placing this into a molding box, and heating to expand this. Specifically, there are expanded foams consisting of urethane, polyolefin or melamine resin. In addition, there are moldings obtained by blending a dissolution material in a polymer, and dissolving out this to form micro pores.
Since the structure obtained by the aforementioned procedure has high porosity, it is widely utilized as a heat insulator, an acoustic material, an adsorbent, a cushioning material or a filter.
Further, in addition to the expanded foams, a spongelike structure obtained by arranging fibers three-dimensionally is also known. Examples of the structure include a structure in which a crimped fiber is formed into a beam structure, and intersecting points of fibers are adhered (see Patent Publication 1).
However, although such the structure has a low apparent density, it is not easy to change a filling density of a fiber in a molding box since fibers are thermally adhered in the state where they are filled into a molding box in order to perform molding, and there is a limit for freely controlling an apparent density. Further, in applications utilizing a specific surface area of a fiber, a smaller number mean diameter of a fiber is required, and as there is the description to the effect that, when a fineness of a single filament is less than 0.5 denier (less than 7 μm in terms of PET specific gravity), bulkiness of the spongelike structure is reduced, in a paragraph [0011] in the Patent Publication, it is difficult to reduce an apparent density in the spongelike structure having a smaller fiber diameter.
For this reason, a spongelike structure in which a fiber diameter is small, an apparent density can be designed depending on an object and an application, and fibers are arranged three-dimensionally, is required.
Additionally, in the field of cell therapy and regenerative medicine, in order to transplant and study a cell, a tissue or an organ, a material which is to be a scaffold for cell cultivating in which a cell is effectively cultivated in vitro, and a material which is to be a scaffold for promoting regeneration or reconstruction of a tissue in vivo (hereinafter, these are collectively referred to as cell culture scaffold) are required. Such the cell culture scaffold material can satisfy various requirements for attaining the aforementioned object, by mimicking the cell environment surrounding a cell.
Meanwhile, in bone marrow or a basement membrane which is one kind of the cell environment surrounding a cell, a cell is grown and proliferated in a three-dimensional matrix called extracellular matrix, such as collagen, constructed of a fibrous structure at a nano-level. For this reason, when a cell is cultivated in vitro for the aforementioned object, previously, a matrix component such as collagen extracted from a living body has been processed into a gel or spongelike structure, and study of this to adopt as a scaffold for three-dimensional cultivating has been progressed (see Patent Publication 2).
However, there are a problem that a biomaterial consisting mainly of a protein can not stand severe treatment, a representative of which is sterile treatment such as autoclave and γ-ray, which is frequently performed in a process for manufacturing a medical material, a problem on stability for long term storage until use, and a problem on a dynamical strength and shape stability. In addition, since a biomaterial such as collagen is generally extracted from an animal such as a cow and a pig, there is a risk that a known or unknown infectious material from these animals, a representative of which is a virus and a prion, is mixed in, and this was a problem upon use as a scaffold material for cultivating a cell in vivo and in vitro, particularly, upon use in medical utility.
For this reason, recently, study of manufacturing a foam or a fibrous material such as a non-woven fabric and a woven fabric using a synthetic polymer in place of a material extracted form a living body, and using them as a three-dimensional cultivating scaffold is being proceeded (see Patent Publication 3-Patent Publication 6).
However, these previous three-dimensional cultivating scaffolds using a synthetic polymer as a material have not an actual shape of a fibrous material called extracellular matrix surrounding a cell in vivo, a representative of which is collagen, particularly a structure mimicking a structure consisting of a fiber at a nano-level. For this reason, they can not be said to mimic the in vivo environment truly, are inferior in affinity for a cell, and influence due to inability to express the cell function as in vivo on the previous scaffold is concerned.
Therefore, in recent years, a structure constructed of a fiber having a diameter at a nano-level (nano-fiber) is paid an attention as a cell scaffold material. For example, many trials to obtain a structure of a nano fiber by a method of blowing a fiber while applying a high voltage, called electrospinning, and cultivating a functional cell, a stem cell or an ES cell used in cell therapy or regenerated medicine on the structure while the function is retained and promoted are performed, and some effect is obtained (see Non-Patent Publications 1 and 2).
However, the structure obtained by such the electrospinning has a defect for use in a scaffold material for cultivating a cell, such as weakness of a fiber strength, ununiformity and scatter of a fiber diameter, and use of an organic solvent upon manufacturing. In addition, since a special process called electrospinning as described above is used, a shape of the resulting structure is limited to a so-called paper-like non-woven fabric structure. For this reason, since the structure as it is has low porosity, and a cell can not enter the interior of a structure, a cell can be cultivated only on a superficial layer, and a cell can not be cultivated three-dimensionally. That is, it is substantially impossible to cultivate a cell at a high density, and it is also impossible to regenerate and reconstruct an organ or a tissue having a thickness in vitro. Further, a non-woven fabric structure also has a defect that a three-dimensional environment in a living body such as bone marrow in which a cell is grown can not be truly reproduced from a view point of a shape.
For this reason, particularly from a view point that a cell or a culture medium can enter the interior of the structure to retain a cell three-dimensionally, and a porous structure and high porosity for passage of a culture medium are possessed, or from a view point of similarity to bone marrow in which many stem cells and hematopoietic cells are grown, as a cell scaffold material for cultivating a cell, a cell scaffold material comprising a spongelike structure consisting of a nano-fiber made of a fibrous material, particularly a synthetic material is sought.
Meanwhile, utilization of a fiber not only as the aforementioned spongelike structure, but also as a filler for a resin, a paint and a cosmetic is progressed. Examples of molding of a fiber into a powder include a fine powder obtained by cutting an ultramicrofiber having a diameter of not more than 3 μm into a length of 5 to 100 μm (see Patent Publication 7). However, since this fiber fine powder is merely dispersion of a fiber into a powder, and is obtained by mechanically grinding an ultramicrofiber after freezing, a fiber is randomly ground and cut in a diameter direction and in a longitudinal direction upon freezing and grinding, and a scatter is great in a fiber length from a view point of a powder. For this reason, there is a problem that, when added as filler for a resin, a paint and a cosmetic, dispersity is inferior due to fibers aggregation and its settlement, storage stability is reduced, and when these are coated, uniform coating is difficult.
For this reason, a powder consisting of a fiber, which is excellent in dispersity and storage stability, and is useful as various fillers is also sought.    Patent Publication 1: Japanese Patent Application Laid-Open (JP-A) No. 9-19580    Patent Publication 2: JP-A No. 62-502936    Patent Publication 3: JP-A No. 62-122586    Patent Publication 4: JP-A No. 2-291260    Patent Publication 5: JP-A No. 7-299876    Patent Publication 6: JP-A No. 2003-265593    Patent Publication 7: JP-A No. 2001-146630    Non-Patent Publication 1: Biomaterials 26 p 5158 (2005)    Non-Patent Publication 2: Tissue Eng. 11 p 1149 (2005)